1) Field of the Invention
The present invention generally relates to a toner image formation apparatus such as an electrophotographic recording apparatus including a toner image carrying body such as a photosensitive drum, a dielectric drum or the like to which a toner image obtained from toner development of an electrostatic latent image is electrostatically adhered and held, and from which the toner image is electrostatically transferred to a recording medium such as a sheet of paper, and in particular, relates to a heat-radiation type toner image fixing device incorporated therein for thermally fusing and fixing the toner image on the recording medium.
2) Description of the Related Art
As a representative of the image formation apparatus as mentioned above, an electrophotographic recording apparatus is well known. In such an apparatus, the following processes are typically carried out:
a) a uniform distribution of electrical charges is produced on a surface of an electrostatic latent image carrying body;
b) an electrostatic latent image is formed on a charged area of the surface of the image carrying body by an optical writing means such as a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array or the like;
c) the latent image is developed as a visible image with a developer or toner, which is electrically charged to be electrostatically adhered to the latent image zone;
d) the developed and charged toner image is electrostatically transferred from the body to a recording medium such as a sheet of paper; and
e) the transferred toner image is fixed and recorded on the paper.
Typically, the electrostatic latent image carrying body may be an electrophotographic photoreceptor, usually formed as a drum, called a photosensitive drum, having a cylindrical conductive substrate formed of a metal such as aluminum, and a photoconductive insulating film bonded to a cylindrical surface thereof and formed of an organic photoconductor (OPC), a selenium photoconductor or the like.
In the toner image fixing process, a heat roller type toner image fixing device is widely used. This device comprises a heat roller and a backup roller, engaged with the heat roller to form a nip therebetween, and a sheet of paper carrying with a toner image is passed through the nip in such a manner that the toner image is in direct contact with the heat roller, whereby the toner image is thermally fused and firmly adhered to the paper by the pressure exerted thereon by the rollers. In this fixing device, the toner image may be subjected to distortion due to the direct contact with the heat roller, especially during a high speed printing.
Another type of fixing device, a heat-radiation type toner image fixing device, is also known. This device is represented by a flash-type toner image fixing device comprising xenon lamps transversely arranged above a path for an sheet of paper carrying with a toner image. The xenon lamps are electrically energized to produce a flash or radiation, when the paper is passed below the xenon lamps. The toner image is thermally fused due to the flash-radiation, and a part of the fused toner penetrates into the fibers of the paper so that the toner image is firmly fixed on the paper. In this type fixing device, since the toner image cannot be directly contacted with any heat element, the quality of the fixed toner image may be superior to that of the toner image fixed by the heat roller type fixing device.
Nevertheless, the flash type toner image fixing device possesses an inherent defect in that a sheet of paper carrying with a toner image may be become undulated upon being subjected to the flash-radiation. When the toner image is unevenly distributed over the surface of the paper, e.g., when the paper includes a forward half zone in which the toner image is evenly recorded, and a rear ward half zone in which no toner image is recorded, an undulation is caused in the paper. In particular, a temperature of the forward half zone or black zone becomes higher than that of the rearward half zone or white blank zone, because a large portion of the flash-radiation is absorbed in the black zone, whereas a large portion of the flash-radiation is reflected from the white blank zone. Accordingly, the amount of moisture in the paper lost at the black zone is larger than that of the paper lost at the white blank zone, so that the black zone of the paper becomes more shrunken than the white blank zone thereof, to thereby cause an undulation in the paper.
Also, for example, when the toner image of the paper includes a table, in which the images of the characters are sparsely recorded, or an illustration, in which white blank zones are included, the paper may be undulated for the same reasons as mentioned above. Note, when the toner image is evenly recorded on the paper, e.g., when the images of characters are recorded on the paper in the full lines thereof, the paper will not be substantially undulated.
Although the undulation of the paper can be removed by leaving it stand in the atmosphere, it is preferable to prevent the paper from being undulated during the fixing process, so that a following process, such as a stacking process can be properly carried out.
In the image formation apparatus as mentioned above, when two-sided recording is performed on the sheet of paper, the undulation of the paper is a problem which must be solved. Where two-sided recording is applied to the paper, after the recording is applied to the first side of the paper, the paper must be reversed and returned to the toner image transferring process for applying the recording to the second side of the paper. In this case, if the paper is undulated, a second toner image cannot be properly transferred from the photosensitive drum to the second side of the undulated paper, because the paper cannot be tightly contacted with the surface of the drum due to the undulation of the paper. Namely, small clearance zones are locally formed between the undulated paper and the surface of the drum, and thus the second toner image cannot be sufficiently transferred to the second side of the undulated paper at the clearance zones.